首先，目前在产品环境可用的MySQL版本（指4.0.x和4.1.x）中，只有InnoDB引擎才允许使用外键，所以，我们的数据表必须使用InnoDB引擎。

下面，我们先创建以下测试用数据库表：


CREATE TABLE `roottb` (
`id` INT(11) UNSIGNED AUTO_INCREMENT NOT NULL,
`data` VARCHAR(100) NOT NULL DEFAULT \'\',
PRIMARY KEY (`id`)
) TYPE=InnoDB;

CREATE TABLE `subtb` (
`id` INT(11) UNSIGNED AUTO_INCREMENT NOT NULL,
`rootid` INT(11) UNSIGNED NOT NULL DEFAULT \'0\',
`data` VARCHAR(100) NOT NULL DEFAULT \'\',
PRIMARY KEY (`id`),
INDEX (`rootid`),
FOREIGN KEY (`rootid`) REFERENCES roottb(`id`) ON DELETE CASCADE
) TYPE=InnoDB;



注意：
1、必须使用InnoDB引擎；
2、外键必须建立索引（INDEX）；
3、外键绑定关系这里使用了“ ON DELETE CASCADE”，意思是如果外键对应数据被删除，将关联数据完全删除，更多信息请参考MySQL手册中关于InnoDB的文档；

好，接着我们再来插入测试数据：


INSERT INTO `roottb` (`id`,`data`)
VALUES (\'1\', \'test root line 1\'),
(\'2\', \'test root line 2\'),
(\'3\', \'test root line 3\');

INSERT INTO `subtb` (`id`,`rootid`,`data`)
VALUES (\'1\', \'1\', \'test sub line 1 for root 1\'),
(\'2\', \'1\', \'test sub line 2 for root 1\'),
(\'3\', \'1\', \'test sub line 3 for root 1\'),
(\'4\', \'2\', \'test sub line 1 for root 2\'),
(\'5\', \'2\', \'test sub line 2 for root 2\'),
(\'6\', \'2\', \'test sub line 3 for root 2\'),
(\'7\', \'3\', \'test sub line 1 for root 3\'),
(\'8\', \'3\', \'test sub line 2 for root 3\'),
(\'9\', \'3\', \'test sub line 3 for root 3\');



我们先看一下当前数据表的状态：


mysql>; show tables;
+----------------+
| Tables_in_test |
+----------------+
| roottb |
| subtb |
+----------------+
2 rows in set (0.00 sec)

mysql>; select * from `roottb`;
+----+------------------+
| id | data |
+----+------------------+
| 1 | test root line 1 |
| 2 | test root line 2 |
| 3 | test root line 3 |
+----+------------------+
3 rows in set (0.05 sec)

mysql>; select * from `subtb`;
+----+--------+----------------------------+
| id | rootid | data |
+----+--------+----------------------------+
| 1 | 1 | test sub line 1 for root 1 |
| 2 | 1 | test sub line 2 for root 1 |
| 3 | 1 | test sub line 3 for root 1 |
| 4 | 2 | test sub line 1 for root 2 |
| 5 | 2 | test sub line 2 for root 2 |
| 6 | 2 | test sub line 3 for root 2 |
| 7 | 3 | test sub line 1 for root 3 |
| 8 | 3 | test sub line 2 for root 3 |
| 9 | 3 | test sub line 3 for root 3 |
+----+--------+----------------------------+
9 rows in set (0.01 sec)



嗯，一切都正常，好，下面我们要试验我们的级联删除功能了。

我们将只删除roottb表中id为2的数据记录，看看subtb表中rootid为2的相关子纪录是否会自动删除：


mysql>; delete from `roottb` where `id`=\'2\';
Query OK, 1 row affected (0.03 sec)

mysql>; select * from `roottb`;
+----+------------------+
| id | data |
+----+------------------+
| 1 | test root line 1 |
| 3 | test root line 3 |
+----+------------------+
2 rows in set (0.00 sec)

mysql>; select * from `subtb`;
+----+--------+----------------------------+
| id | rootid | data |
+----+--------+----------------------------+
| 1 | 1 | test sub line 1 for root 1 |
| 2 | 1 | test sub line 2 for root 1 |
| 3 | 1 | test sub line 3 for root 1 |
| 7 | 3 | test sub line 1 for root 3 |
| 8 | 3 | test sub line 2 for root 3 |
| 9 | 3 | test sub line 3 for root 3 |
+----+--------+----------------------------+
6 rows in set (0.01 sec)



嗯，看subtb表中对应数据确实自动删除了，测试成功。

结论：在MySQL中利用外键实现级联删除成功！